With the increase of Internet traffic, larger capacity is needed in the optical communication system of the trunk system. When the bit rate per wavelength increases, the chromatic dispersion, polarization mode dispersion, and waveform distortions of various non-linear effects in the transmission path result in serious degradation in the quality of the information.
Compared with the incoherent technology, the digital coherent reception technology has the following advantages: it has approximately 3 dB optical signal to noise ratio (OSNR) gain; it can easily adopt the electronic equalization technique to cope with channel variations and reduce costs; it can use more efficient modulation techniques and polarization multiplexing to increase the transmission capacity. Thus the digital coherent technology is considered as a key technology in the high-speed optical communication system.
In the optical coherent receiver, the signal light and the local oscillation light are mixed, amplitude information and phase information of the signal light are moved to the baseband signal, thus the optical coherent detection retains all the information of the light field and can take the advantages of the digital signal processing technology in features and performance. Using the electric equalization technology can almost completely compensate for linear distortions of the optical signal, such as compensating the chromatic dispersion (CD), the polarization mode dispersion (PMD), and the like.
FIG. 1 (a) is a block diagram of a typical digital coherent receiver. The received optical signal is split into two mutually orthogonal polarization signals by a polarization beam splitter (PBS). The polarized signal output by the PBS is mixed with a local oscillation optical signal through a 90° optical mixer. The mixed optical signal is converted to a baseband electrical signal through a balancing photoelectrical detector. The photoelectrically-converted electrical signal has two signals for each polarization state, but the four signals do not correspond to the original four signals, this is because, after passing through the transmission channel, there is crosstalk between the two polarization states, and the polarization states rotate, the two polarization states at the receiving ends, the two orthogonal signals of each polarization state and the transmitted signals do not have a correspondence. The photoelectrically-converted electrical signal is converted into a digital signal by the ADC. The general digital signal processing technology can be used to process the ADC converted digital signal.
The digital signal processing part, as shown in FIG. 1 (b), comprises the IQ Skew compensation, the IQ imbalance compensation, chromatic dispersion (CD) compensation, dispersion estimation, clock recovery, polarization demultiplexing, carrier recovery, judgment detection. The IQ Skew adjusts delays of the 4 signals, and solves the inconsistencies in the delays of the 4 signals at the photoelectric conversion front end. The chromatic dispersion (CD) compensation unit often uses the sampling frequency domain fast convolution technique to compensate the chromatic dispersion of the optical link. The clock recovery adjusts the sampling phase, and provides data with a stable sampling phase for the polarization demultiplexing module thereafter. The polarization demultiplexing compensates the residual dispersion and polarization mode dispersion (PMD). The carrier recovery compensates for inconsistencies in frequencies of the local oscillation light at the receiving end and the carrier light at the sending end. Wherein the value of the chromatic dispersion (CD) is generally large, the equalization of the chromatic dispersion and the polarization mode dispersion (PMD) is generally completed in two parts, first, static dispersion is compensated, wherein the equalizer usually cannot use a standard adaptive algorithm to update coefficients, such as compensating 40000 ps/nm chromatic dispersion, the number of filter taps should reach big hundreds or even thousands, and typically the fast Fourier transform technology is used for frequency domain fast convolution. The dispersion estimation module provides the dispersion compensation module with dispersion value to be compensated.
The compensation of the residual chromatic dispersion and polarization mode dispersion is achieved by a FIR butterfly equalizer, wherein the FIR butterfly filter uses an adaptive algorithm to update the coefficients to track and compensate the polarization mode dispersion that dynamically changes over time. The FIR butterfly equalizer is also known as polarization demultiplexing. The FIR butterfly adaptive equalizer has the functions of equalizing, matching and filtering, and sampling position adjusting. When the sampling position change range is too large, or the sampling frequency offset exists, the sampling phase change range exceeds the adjustment range of the FIR butterfly adaptive equalizer, it will cause the FIR butterfly adaptive equalizer to not work properly. It needs to place a clock recovery module before the FIR butterfly equalizer.
The clock recovery estimates the sampling time error of an input symbol, and performs interpolation adjustment on the sampling time of the symbols, or adjusts the ADC sampling frequency via the VCO, to guarantee to provide a stable symbol sampling phase. The clock recovery phase detector needs to tolerate a certain signal distortion, while the conventional clock recovery usually can only tolerate small dispersion. In order to not increase the complexity of the clock recovery module, while currently there is a lack of clock recovery method that tolerances large residual dispersion value, it requires an accurate dispersion compensation.
In summary, with the method proposed in the related art for performing feedback control of a variable dispersion compensator by using the transmission quality information (e.g., error rate, Q factor, etc.), a dispersion interval with a certain step size is used to change the dispersion compensation amount of a dispersion compensation filter, until the system converges. When the system starts up, the search process is slow and the accuracy is low. Moreover, the optical fiber link is affected by the environmental temperature change, and the link dispersion value also changes slowly. In addition, in the working process, with the search based dispersion estimation method in the related art, it is difficult to judge a slow change of the dispersion value in the operation.